1. Field of the Invention
The present invention relates to a thin film capacitor and a manufacturing method thereof, and more particularly to a thin film capacitor used for ICs and LSIs and a manufacturing method thereof.
2. Description of the Related Art
The electronic circuits are being miniaturized to an increasing degree as a result of advances in the integrated circuit technologies, and the miniaturization of capacitors that are indispensable in the integrated circuits as circuit elements for various kinds of electronic circuits is also becoming especially significant. The miniaturization of thin film capacitors is being delayed in the midst of a rapid progress in the miniaturization of the active elements, which is becoming a substantial factor as an obstruction for a further enhancement in the integration. The reason for this is that the dielectric thin films are limited to those materials such as SiO.sub.2, Si.sub.3 N.sub.4 and the like that have dielectric constant of less than 10. Therefore, it is becoming necessary to develop dielectric thin films having a large dielectric constant as a means of the miniaturization of thin film capacitors. It has been known that BaTiO.sub.3, SrTiO.sub.3 and PbZrO.sub.3 which are perovskite type oxides and an ilmenite type oxide LiNbO.sub.3 that are represented by the chemical formula ABO.sub.3, or oxides belonging to ferroelectric materials such as Bi.sub.4 Ti.sub.3 O.sub.12, have dielectric constant of larger than 100 and reaching even to 10000 in the form of a single crystal or a ceramic as the above-mentioned single composition and the mutual solid solution composition, and these oxides are being used widely for ceramic capacitors. To make these materials into thin films is extremely effective for the miniaturization of the above-mentioned thin film capacitors, and it has been studied since fairly long time ago. An example with relatively satisfactory characteristics among these researches is an article reported in "Proceedings of the IEEE", Vol. 59, No. 10, 1971, pp. 1440-1447 in which it is shown that dielectric constants of 16 (formed at room temperature) to 1900 (with a heat treatment at 1200.degree. C.) are obtained for BaTiO.sub.3 thin films that are formed by sputtering and subjected to heat treatments.
Dielectric thin films, such as the above-mentioned BaTiO.sub.3, that have been formed in the past require high temperatures at the time of thin film formation in order to obtain high dielectric constants, being formed without exception on a lower electrode of a high-melting point noble metal such as platinum and palladium. The reason for doing so is that the dielectric constant of the dielectric films on aluminum, nichrome, copper and the like that are generally used as the electrode materials is reduced markedly due to the oxidation of the electrode at high temperatures or their mutual reactions with the dielectric films.
On the other hand, the electrode material used extensively for the present day highly integrated circuits is polycrystalline silicon or a low resistance silicon layer with impurities doped to a high concentration in a portion of the silicon substrate itself. In the description that follows these materials will generically be called silicon electrodes. For the silicon electrodes, the fine processing technologies have been established and have already been used extensively. Therefore, if a thin film with satisfactory high dielectric constant can be prepared on a silicon electrode, it becomes possible to apply the technology to capacitors for integrated circuits. However, it is reported that when a thin film of a high dielectric constant material is formed on silicon in accordance with the conventional technology, there will be formed in an interface a layer which is equivalent to silicon dioxide (SiO.sub.2) of about 100 .ANG., for example, on pages 687 to 688 in an article on a SiTrO.sub.3 film, IBM Journal of Research and Development, November issue, 1969, pp. 686-695. The interface layer has a low dielectric constant, and as a result, the effective dielectric constant of the high dielectric constant film formed on silicon is reduced markedly, practically impairing the advantage of the use of the material with high dielectric constant. Another example of a similar conclusion can be found on p. 316 of an article on BaTiO.sub.3 published in Journal of Vacuum Science and Technology, Vol. 16, No. 2, 1979, pp. 315-318.
From an analogical inference of the above-mentioned prior art, there can be considered a means of further employing a platinum or palladium electrode between the high dielectric constant material film and the silicon electrode also in the case of using a silicon electrode as the lower electrode. The structure for this case is shown in FIG. 1. Reference numeral 11 is a silicon substrate, 12 is a low-resistance silicon electrode layer doped with a high concentration impurity such as phosphorus or arsenic, 13 is an insulating layer of such material as SiO.sub.2, 14 is a platinum electrode layer, 15 is a dielectric layer, 16 is an upper electrode layer of such material as aluminum and 17 is a lead-out wiring for the lower electrode. However, the formation of the high dielectric constant material film represented by BaTiO.sub.3 and SrTiO.sub.3 has to be done at high temperatures as mentioned above. Accordingly, with the means shown in FIG. 1, the platinum layer 14 reacts with the silicon electrode 12 forming platinum silicide, and further, there is formed an SiO.sub.2 layer on its interface with the dielectric layer 15, and as a result, the effective dielectric constant is markedly reduced analogous to the case of the silicon electrode described in the above.